Why am I maturing so much slower than literally everyone in my grade except for like two people??

I'm short, I'm light, I look like a freaking 8 year old.

Even my cousin, who is a year younger than me, has matured more than me.

My friends that are two years younger than me are more physically matured!!!

(I'm talking about physical maturity. As for mental maturity... I'm probably the most mature of all my friends)